One of the developmental areas involving data transmission is the use of frequency-hopped networks which have been designed to provide for secure, reliable digital communications. These systems are generally able to maintain intelligible communication with as much as 20% of their channels jammed, and for this reason it is possible to successfully transmit data in noisy or Jammed spectral regions.
Frequency-hopped radio transmissions create processing gain by utilizing a large number of independent frequency locations. For example, certain types of radio transmitters make use of up to 2,000 different hop locations. It follows that the input bandwidth, W, of a frequency-hopped signal detector is much larger than the width of the binary phase shift keying (BPSK) envelope.
Spectral analysis techniques will not always reveal the presence of hybrid FH/DS (frequency-hopping/direct sequence) signals because of the inherent covert nature of these signals. However, certain classes of Fourth Law detectors have been shown to be useful against all types of frequency-hopped signals. The class of channelized detectors have been shown to be useful as well, owing to their selective elimination of narrowband interference sources.
The class of hybrid detectors, which combine channelizing and nonlinear combining techniques with Fourth Law Detectors, has been shown to be especially useful for frequency-hopped signal detection. These detectors utilize channelizing techniques such as those discussed in U.S. patent applications U.S. Ser. No. 417,175 entitled "Channelized Binary-Level Hop Rate Detector", and U.S. Ser. No. 417,124 entitled "Channelized Binary-Level Radiometer". These applications are related to the present disclosure and are included herein by reference. These applications are also commonly assigned to the same corporate entity, the Unisys Corporation of Blue Bell, Penn.
One of the problems in frequency-hopped data transmission detection is the determination of the "chip rate" or "data rate" of a covert frequency-hopped/direct sequence (FH/DS) signal. Very often, narrowband interference signals make this detection process difficult unless the detector is provided with considerable immunity to narrowband interference.
The present disclosure incorporates delay and mix techniques with channelizing and non-linear combining techniques to create a chip rate detector which can be used for the detection and characterization of covert frequency-hopped/direct sequence signal transmissions.
In direct sequence (DS) spread spectrum communications, for each information data bit (i), there are "N" transmitted coded bits, each coded bit called a "chip" (c), thus giving the ratio N=R.sub.c /R.sub.1. This ratio represents the factor by which the original bandwidth is spread. R.sub.c is the chip rate (coded) while R.sub.1 is the information bit rate.
The coded bit (chip) rate R.sub.c is larger than the information bit rate R.sub.1 since for each information bit there are N coded bits (chips) which provide the relation that N=R.sub.c /R.sub.1.
Suppose that for the transmission of an uncoded information sequence with the information bit rate of R.sub.1, a bandwidth of B.sub.1 is required. By increasing the bandwidth to B.sub.1 (B.sub.c =NB.sub.1), it is possible to transmit in the extended bandwidth, a coded bit rate of R.sub.c =NR.sub.1.
In direct sequence (DS) spread spectrum communications, N coded bits are transmitted for every information data bit each time in a differently coded form. In order to distinguish it from the data bit, each of the N coded bits is known as a "chip". The coded signal is achieved by modulo-two addition of a sequence of N chips for each of the data bits. The result of this addition is a chip rate of R.sub.c is used to modulate the carrier frequency.
While the data stream depends on the information bits being transmitted, the "chip" stream is generated using a predetermined coding scheme. That is, a stream pattern of N chips is transmitted for each data bit.
The chip stream pattern of N chips per bit can take any sequence form, of "ones" and "zeroes", provided that it can be reproduced identically for each information bit, and that it is available both in the transmitter and receiver.
The advantage of frequency hop (FH) signals is twofold. First, they have a high degree of immunity to narrowband interference and can, therefore, be operated in dense interference environments.
Second, frequency hop signals are relatively covert in that the signal power is spread over a much greater bandwidth than is required by the data rate. Due to these properties, frequency hop detectors must be able to detect signals at a low signal-to-noise ratio while operating in the presence of multiple narrowband interference signals.
The present disclosure presents a hybrid FH/DS chip rate detector which, due to channelization and a unique non-linear combining technique, provides improved performance over currently known detectors and is relatively immune to the effects of narrowband interference.
Another term useful in these types of data transmissions is the concept of "hop rate". This represents the rate at which the transmitted signal carrier frequency hops from one frequency to another frequency.
There is seen in FIG. 3 a schematic drawing of a delay and mix chip rate detector. This type of detector is commonly used for generating a spectral line at the chip rate (R.sub.c) of a carrier-modulated phase shift keying (PSK) signal. The PSK signal, designated as x(t), at the input of the detector, is given by equation EQ I as follows: EQU x(t)=Ae-.sup.jw.sbsp.c.sup.t+.PHI.(t) EQ I
where A is the amplitude and .PHI.(t) is the piecewise constant phase of the baseband PSK signal, and w.sub.c is the carrier frequency, and j=.sqroot.-1. This signal is then multiplied by the complex conjugate of a delayed copy of itself to obtain equation EQ II as follows: EQU y(t)=A.sup.2 e.sup.-j[.PHI.(t)-.PHI.(t-T.sbsp.c.sup./2)+w.sbsp.c.sup.T.sbsp.c.sup./2]EQ II
where T.sub.c is the time between chips of the PSK signal.
The output, y(t), can be considered to be the sum of a random phase signal and a deterministic square wave signal.
During every other T.sub.c /2 period, the phases .PHI.(t) and .PHI.(t-T.sub.c /2) are equal and cancel each other, making y(t) equal to the complex constant of equation EQ III as follows: EQU y(t)=A.sup.2 e.sup.-jw.sbsp.r.sup.T.sbsp.c.sup./2 EQ III
During the other time periods (other than the previously mentioned phases, however, the phases .PHI.(t) and .PHI.(t-T.sub.c /2) are random and independent, making y(t) equal to equation EQ IV, as follows: EQU y(t)=A.sup.2 e.sup.-j.theta. EQ IV
with .theta. being a random phase.
The "random signal" consists of the "chips" of length T.sub.c /2 and the random phase, spaced one chip-time apart.
The "deterministic signal" consists of the constant phase chips of length T.sub.c /2 spaced one chip-time apart. The deterministic component is periodic with a period T.sub.c and has a strong fundamental frequency component at the time period 1/T.sub.c.
Therefore, spectral analysis of the output of the delay and mix detector of FIG. 3 will reveal a spectral line at the chipping rate when a PSK signal is present at the input.
In FIG. 3, the PSK signal x(t) is shown on line 10x at the input of the detector. The complex conjugate of a delayed copy of itself is provided by the circuit 14 of FIG. 3, which circuit provides its output to a multiplier unit 16 which then combines the inputs to the multiplier to form the output signal y(t) on line 18y.
In this system of FIG. 3, the carrier of the PSK signal remains constant. However, in "hybrid" FH/DS (frequency-hopped/direct sequence) systems, the carrier of the PSK signal is hopped over a multitude of frequencies. Thus a "delay and mix" detector with an input bandwidth equal to the direct sequence (DS) bandwidth may be used on such signals. However, the probability of intercept will be inversely proportional to the processing gain of the frequency-hopped system and the detector performance may be unacceptable.
There are other delay and mix detectors, such as the MODAC, which operate on the entire hop bandwidth. The MODAC was developed by the Pacific Sierra Research Company located in Los Angeles, California. However, these types of detectors are vulnerable to the narrowband interference often accompanying frequency-hopped (FH) signals.
The present disclosure which involves a "channelized" delay and mix detector (CDMD) also operates on the entire hop bandwidth but reduces the effects of narrowband interference by "channelizing" the signal and performing a parallel set of delay and mix detectors as will be discussed in connection with FIG. 2 and 3 hereinafter.